1. Field of the Invention
The present invention relates to a thermoelectric material for high temperature use which is used for thermoelectric power generation based on the Seebeck effect or a so-called Peltier's thermoelectric effect of electronic freezing and the like effect of direct energy conversion without using a movable part, and its production method.
2. Related Art Statement
Thermoelectric conversion, such as, thermoelectric power generation and electronic freezing, using a thermoelectric material allows production of a simplified direct energy conversion apparatus of a simple, reliable, long serviceable and maintenance-easy structure not having a movable part which is a cause of vibration, noise and abrasion, etc. It is suited to direct generation of a direct current by combustion of various fossil fuels or temperature control without using a cooling medium.
In evaluating characteristic properties of a thermoelectric conversion material, an electric power factor Q or a Figure-of-Merit Z are used which are expressed by the following formulae. EQU Q=.sigma..alpha..sup.2 ##EQU1## wherein, .alpha. is Seebeck coefficient, .sigma. is an electric conductivity, and K is a thermal conductivity. In thermoelectric materials, a large Figure-of-Merit Z is desired. Namely, a higher Seebeck coefficient, a higher electric conductivity .sigma., and a higher thermal conductivity are desired.
In case of using a thermoelectric material for thermoelectric power generation, the thermoelectric material is required to have a high Figure-of-Merit Z of at least Z=3.times.10.sup.-3 /K and function stably for a long time in the environmental atmosphere. In case of mass producing a thermoelectric material for automobile vehicle use or waste heat utilization purpose, a thermoelectric material having a non-degradable character, a sufficient strength and heat durability at high temperatures is particularly desired, as well as an economical and efficient production method thereof. Heretofore, as such a thermoelectric material, use has been made of Pb, Te, silicide compounds, such as, MSi.sub.2 (wherein, M is Cr, Mn, Fe or Co), etc., or mixtures thereof.
Examples of thermoelectric materials having a Sb compound, such as TSb.sub.3 (wherein, T is Co, Ir or Ru), for example, CoSb.sub.3 as a main component and an impurity added for deciding the electric conductivity type are described in the following literatures.
1) L. D. Dudkin and N. Kh. Abrikosov, Soviet Physics Solid State Physics (1959) pp.126-133
2) B. N. Zobrina and, L. D. Dudkin, Soviet Physics Solid State Physics (1960) pp.1668-1674
3) K. Matsubara, T. lyanaga, T. Tsubouchi, K. Kishimoto and T. Koyanagi, American Institute of Physiks (1995) pp.226-229.
A thermoelectric material consisting of PbTe has a large Figure-of-Merit Z of about 1.times.10.sup.-3 /K at around 400.degree. C., however, it contains a volatile component Te in the raw material composition, so that it has a low melting point and a weak chemical stability and hence cannot be used at high temperatures of exceeding 500.degree. C. Moreover, because the thermoelectric material contains the volatile matter Te and hence its production process is rather complicated, its characteristic properties are liable to fluctuate due to fluctuation of the composition, so that efficient mass production is impossible. In addition, the raw materials per se of the thermoelectric material are expensive and highly noxious.
Meanwhile, silicide compounds, such as, MSi.sub.2 (M=Cr, Mn, Fe, Co) and silicide series materials, such as, mixtures of the silicide compounds can be produced from cheap and innoxious raw materials, and they are chemically stable and can be produced even at a temperature region of about 800.degree. C. For example, they can be produced by a relatively economical production method as described in Isao Nishida and Kinichi Kamimura "Thermoelectric materials and their applications" pp. 176-180 (1983). However, silicide series materials could not have sufficient thermoelectric properties comparable to PbTe in that their Figures-of-Merit Z are about (1-2).times.10.sup.-4 /K which are one order smaller than PbTe.
Thermoelectric materials TSb.sub.3 (T=Co, Ir, Ru) and the like Sb compounds, such as, thermoelectric materials composed mainly of CoSb.sub.3, can be produced from cheap and innoxious raw materials and are known to have relatively high Figures-of-Merit Z of &lt;1.times.10.sup.-3 /K.
It is known that, in the heretofore known thermoelectric materials having CoSb.sub.3 in their chemical compositions, the produced materials should be composed solely of a crystal phase CoSb.sub.3 of cubic crystal system and the other crystal phases CoSb, CoSb.sub.2 and Sb should be removed for the sake of their function of decreasing the thermoelectric properties. However, in practice, in the method of producing CoSb.sub.3 by melting, it is known that foreign phases COSb, CoSb.sub.2 and Sb other than CoSb.sub.3 are precipitated at the time of solidification, and a heat treatment of a temperature of around 600.degree. C. for about 200 hrs is needed for converting the melted materials to the sole crystal phase of CoSb.sub.3, so that a long time-consuming production process is required.
In a prior method of producing CoSb.sub.3 by pulverizing and sintering melted materials of CoSb.sub.3, foreign crystal phases CoSb and CoSb.sub.2 precipitated at the time of melting and having higher densities than CoSb.sub.3 are phase changed to CoSb.sub.3 at the time of sintering, so that a problem of volumetric expansion arises to obstruct the proceeding of the sintering. For example, even when the melted materials are hot pressed at a condition of a pressure of 5.times.10.sup.3 kg/cm.sup.2 and a temperature of 600.degree. C., a sufficiently densified material could not been obtained (refer to K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Shimamoto and T. Kobayashi "American Institute of Physics" pp. 226-229 (1995), wherein the density of CoSb.sub.3 of cubic crystal system is reported as 5.25 g/cm.sup.3 at the maximum, whereas the theoretical density of CoSb.sub.3 is 7.64 g/cm.sup.3) As a result, the produced sintered body is very brittle and has an insufficient strength at high temperatures.
Therefore, in order to apply a thermoelectric material to electric power generation apparatuses of automobile cars using waste heat of the automobile cars, a thermoelectric material has been earnestly desired which is chemically stable and hardly deteriorated on its characteristic properties and has a sufficient heat resistant property at high temperatures of not less than 600.degree. C. Particularly, a thermoelectric material for high temperature use has been eagerly desired which has a high strength and thermoelectric properties at high temperatures.
An industrial method of economically and efficiently producing such a thermoelectric material for high temperature use, has also been desired.